1. Field of the Invention
The invention relates to an inverter for driving multiple lamps in an LCD display. More specifically, the invention relates to the magnetic coupling of inductors and magnetic coupling of output transformers for each of the multiple lamp driver circuits.
2. Description of the Related Art
An LCD based monitor general needs efficient and low profile backlighting for information display. The narrow diameter cold-cathode fluorescent lamp (CCFL), for example the T1 type by Philips, is widely used in the industry. With the increase of monitor size, multiple lamps are needed for the panel illumination. To drive these CCFLs, high frequency electronic ballasts with high efficiency and low profile are in demand. Due to its low losses and low stresses, the voltage-fed half-bridge resonant converter is used to drive the CCFL and other fluorescent lamps. In developing the electronic inverter for multiple CCFLs, people usually prefer to use one single inverter instead of two or more in order to reduce cost and circuit complexity. In this endeavor, the so-called series structure in FIG. 1 and the parallel structure in FIG. 2 are currently used for dual lamp inverters. Comparing these two structures, one can have the following observations.
The series structure in FIG. 1 has a) better lamp current matching due to the series connection of the output transformer primary sides, b) less (3) magnetic components. However, it has higher output transformer turns ratio, which translates to higher primary side winding current, and to more conduction losses. Also, when the output transformer secondary side winding turns increase, the wire size needs to be reduced (e.g. 44AWG) such that the wire fits in the given window area. In addition to contributing to higher conduction losses in the winding, the small size of the wire may cause problems during the manufacturing process.
On the other hand, the parallel structure of FIG. 2 can use a lower turns ratio output transformer. In addition to clear modularity, the secondary side leakage inductance can be reduced and the system performance is improved. However, the parallel structure in FIG. 2 suffers from poor lamp current matching and requires more (4) magnetic components for dual lamps. What is needed is a magnetic component integration approach to overcome the shortfalls of the parallel structure.